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Air pressure and atmospheric motion

Explain the forces that generate winds Explain why there is well-developed westerly flow in the upper troposphere Differentiate between high and low pressure systems Compare and contrast surface and geostrophic winds Describe/read maps of constant pressure surfacesIdentify and discuss wind types and the forces that generate them Recognize surface and upper-level atmospheric maps and identify general patterns of windflow Explain monsoonal flow and land-sea breezes

air pressure and atmospheric motion
Air pressure and atmospheric motion

Q: What makes the wind blow?

A: Air pressure differences.

air pressure
Air pressure
  • Force exerted by molecules in atm due to gravity and temperature
surface air pressure variability
Surface air pressure variability
  • Average sea level pressure 1013 mb

Mercury barometer

1013 mb = 29.91 inches

Aneroid barometer

pressure systems
Pressure systems
  • Two types: high and low
  • Low: associated with clouds and instability.
  • High: associated with clear conditions and stability
low pressure systems
Low pressure systems
  • Cyclone
  • Converging rising air at surface
  • Diverging air aloft
  • Winds rotate counterclockwise in NH
  • Areas of “light” atmosphere; air is forced into these locations
  • Unstable surface conditions
high pressure system
High pressure system
  • Anticyclone
  • Converging air aloft
  • Diverging sinking air at surface
  • Winds rotate clockwise in NH
  • Areas of “heavy” atmosphere; air is forced out of these locations
  • Stable surface conditions
high and low pressure systems
High and low pressure systems
  • Occur on a variety of spatial and temporal scales
    • Some pressure systems may be stationary for a long period of time, others may migrate rapidly around the planet
    • Some pressure systems are closed, others are more belt-like and open
low pressure systems1
Low pressure systems
  • Types of low pressure systems: tornados, thunderstorms, hurricanes….
low pressure systems2
Low pressure systems

….., midlatitude cyclones, the ITCZ, thermal lows

another secret of weather forecasting
Another secret of weather forecasting
  • The atmosphere is a heterogeneous collection of pressure systems in three dimensions.
  • Weather forecasting involves looking at surface conditions as well as upper level conditions (aloft, in the upper troposphere)
deep tropospheric phenomena
“Deep” tropospheric phenomena
  • A midlatitude cyclone is a low pressure at the surface coupled to a low pressure aloft in the upper troposphere
  • A hurricane is a high pressure aloft and a low pressure at the surface
midlatitude cyclones
Midlatitude cyclones
  • Strong, “deep” interaction between surface and upper levels
  • May travel large distances around the globe

Midlatitude cyclone

shallow tropospheric phenomena
“Shallow” tropospheric phenomena
  • Thermal low (warm)
  • Thermal high (cold)
    • Weak interaction between surface and upper levels
    • May occur on a daily basis or persist over many months



  • Named according to the direction they blow from
  • Winds can blow at different directions at different altitudes in the atm
  • Forces that act on winds:
    • PGF
    • CF
    • Surface friction
2 coriolis force cf
2. Coriolis Force (CF)
  • Apparent deflection of the winds due to rotation of the Earth
    • NH winds deflected right
    • SH winds deflected left
coriolis force cf
Coriolis Force (CF)
  • CF is not a true force; it is an apparent force arising from the effect of the Earth’s rotation
  • Deflection is strongest at poles and zero at the Equator
  • CF acts perpendicular to the direction of motion
  • CF deflects to the right in the Northern Hemisphere
misconceptions about the coriolis force
Misconceptions about the Coriolis Force
  • The CF does not determine the rotation in a drain.
3 surface friction sf
3. Surface friction (SF)
  • Topography (mnts, elevated plateaus) deflect winds
two major types of winds
Two major types of winds
  • Geostrophic (upper troposphere winds)
    • Influenced by PGF and CF only
    • Wind flow is parallel to isobars/geopotential heights
    • Geostrophic flow is westerly (west to east) in NH
    • Shown on geopotential height maps
two major types of winds1
Two major types of winds

2. Surface winds

  • Influenced by PGF, CF, and SF
  • Winds cross isobars
types of surface winds
Types of surface winds
  • Monsoonal flow
    • Creates extreme wet and dry seasons

Location of

thermal highs

and lows associated

with monsoonal


Monsoon in India




Weak monsoonal flow in southwestern US

types of surface winds1
Types of surface winds
  • Land-sea breeze circulation
  • Many other different types of surface winds based on local physiography and arrangement of pressure cells. Many have unique names.

Sea breeze blowing

from ocean to land